1. Field of the Invention
This invention relates to a low-frequency induction heater utilizing a one-turn transformer s an electromagnetic induction heat generator and, more particularly, to a low-frequency induction heater which comprises a secondary conductive hollow cylindrical member of a sole stainless steel material.
2. Background of the Invention
Heretofore, an electric fryer has been proposed which comprises an oil container, a pipe-like portion formed substantially in a central portion of the oil container, and an induction heater inserted in the pipe-like portion with a gap provided by means of positioning ridges, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 39,525/1983.
Meanwhile, the inventor of the present invention earlier proposed a low-frequency electromagnetic induction heater, comprised of an induction coil wound on a core, and a single metal pipe or two or more different metal pipes combined into an integrated structure around the induction coil, where the gap between the induction coil and the pipe or pipes is filed with a resin molding, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 297,889/1990.
However, in the former heater, i.e., the electric fryer, heat generated from the induction heater is transferred to the pipe-like portion, which is a part of the oil container, through the air gap between the induction heater and the pipe-like portion which causes the problem of low heat transfer efficiency. Therefore, when oil in the container is heated to the cooking temperature necessary for cooking fries, tempuras, or the like, the temperature of the induction heater is raised to a considerably high temperature, which has an adverse effect on the coil and the core of the induction heater. Particularly, the temperature of the induction heater is liable to exceed the acceptable temperature limit of the coil insulator.
In the latter heater, i.e., the low-frequency electromagnetic induction heater, the secondary winding is a part of the container. Thus, Joule heat is generated by electromagnetic induction in the container. This achieves the advantage that satisfactory energy transfer efficiency can be obtained by avoiding an excessive temperature rise in the coil and the core. However, where the secondary winding uses; a combination of copper, which has a low electric resistivity, and stainless steel which is durable, i.e., where a copper pipe and a stainless steel pipe (a part of the vessel) are combined into an integrated structure, the heater has the following disadvantages:
(1) When the overall pipe structure is heated and the temperature is raised, the difference in the coefficient of thermal expansion between the two metals causes circumferential elongation of the copper pipe relative to the stainless steel pipe, in other words, a portion of the circumference of the copper pipe expands inward to produce an air gap between the inner copper pipe and the outer stainless steel pipe. In the portion where the air gap is produced, the heat transfer efficiency deteriorates and localizes the temperature rise, which causes oxidation of the copper. FIGS. 11(a) and 11(b) are sectional views showing gap formation. Before the temperature rise, the copper pipe 21 and the stainless steel pipe 22 are perfectly integrated FIG. 11(a)). After the temperature rise, however, an air gap 23 is formed between the two pipes 21 and 22 (FIG. 11(b)).
(2) If a leakage current, e.g., a grounding current, is caused in the heater while there is water on the contact portions of the copper and stainless steel pipes, electrolytic corrosion occurs about which deteriorates the secondary winding.
(3) For combining the copper pipe and stainless steel pipe into an integrated structure, high dimensional accuracy is required in the shapes of both the pipes, and this inevitably leads to increased manufacturing cost.
(4) In case the integrated structure of a copper pipe and a stainless steel pipe is used as a secondary winding, the number in layers of a primary winding increases from 4 layers to 6 layers. This means that heat dissipation from the inside of the primary winding is difficult, which eventually causes overheating in the primary winding.